This invention relates in general to xerography and, more specifically, to a novel photosensitive device and method of use.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated by U.S. Pat. No. 3,121,006 to Middleton and Reynolds which describes a number of binder layers comprising finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In its present commercial form, the binder layer contains particles of zinc oxide uniformly dispersed in a resin binder and is coated on a paper backing.
In the particular examples described in Middleton et al, the binder comprises a material which is incapable of transporting injected charge carriers generated by the photoconductor particles for any significant distance. As a result, with the particular material disclosed in Middleton et al patent, the photoconductor particles must be, in substantially continuous particle-to-particle contact throughout the layer in order to permit the charge dissipation required for stable cyclic operation. Therefore, with the uniform dispersion of photoconductor particles described in Middleton et al, a relatively high volume concentration of photoconductor, about 50 percent by volume, is usually necessary in order to obtain sufficient photoconductor particle-to-particle contact for rapid discharge. However, it has been found that high photoconductor loadings in the binder results in the physical continuity of the resin being destroyed, thereby significantly reducing the mechanical properties of the binder layer. Systems with high photoconductor loadings are often characterized as having little or no flexibility. On the other hand, when the photoconductor concentration is reduced appreciably below about 50 percent by volume, the photo-induced discharge rate is reduced, making high speed cyclic or repeated imaging difficult or impossible.
U.S. Pat. No. 3,121,007 to Middleton et al teaches another type of photoreceptor which inclues a two-phase photoconductive binder layer comprising photoconductive insulating particles dispersed in a homogeneous photoconductive insulating matrix. The photoreceptor is in the form of a particulate photoconductive inorganic pigment broadly disclosed as being present in an amount from about 5 to 80 percent by weight. Photodischarge is said to be caused by the combination of charge carriers generated in the photoconductive insulating matrix material and charge carriers injected from the photoconductive pigment into the photoconductive insulating matrix.
U.S. Pat. No. 3,037,861 to Hoegl et al teaches that poly(vinylcarbazole) exhibits some long-wave U.V. sensitivity and suggests that its spectral sensitivity be extended into the visible spectrum by the addition of dye sensitizers. Hoegl et al further suggest that other additives such as zinc oxide or titanium dioxide may also be used in conjunction with poly(vinylcarbazole). In Hoegl et al, the poly(vinylcarbazole) is intended to be used as a photoconductor, with or without additive materials which extend its spectral sensitivity.
In addition to the above, certain specialized layered structures particularly designed for reflex imaging have been proposed. For example, U.S. Pat. No. 3,165,405 to Hoesterey utilizes a two layered zinc oxide binder structure for reflex imaging. The Hoesterey patent utilizes two separate contiguous photoconductive layers having different spectral sensitivies in order to carry out a particular reflex imaging sequence. The Hoesterey device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.
It can be seen from a review of the conventional composite photoconductive layers cited above, that upon exposure to light, photoconductivity in the layer structure is accomplished by charge transport through the bulk of the photoconductive layer, as in the case of vitreous selenium (and other homogeneous layered modifications). In devices employing photoconductive binder structures which include inactive electrically insulating resins such as those described in the Middleton et al, U.S. Pat. No. 3,121,006, conductivity or charge transport is accomplished through high loadings of the photoconductive pigment allowing particle-to-particle contact of the photoconductive particles. In the case of photoconductive particles dispersed in a photoconductive matrix, such as illustrated by the Middleton et al U.S. Pat. No. 3,121,007, photoconductivity occurs through the generation and transport of charge carriers in both the photoconductive matrix and the photoconductor pigment paricles.
Although the above patents rely upon distinct mechanisms of discharge throughout the photoconductive layer, they generally suffer from common deficiencies in that the photoconductive surface during operation is exposed to the surrounding environment, and particularly in the case of repetitive xerographic cycling where these photoconductive layers are susceptible to abrasion, chemical attack, heat and multiple exposures to light. These effects are characterized by a gradual deterioration in the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of persistent conductivity which fail to retain an electrostatic charge, and high dark discharge.
In addition to the problems noted above, these photoreceptors require that the photoconductor comprise either a hundred percent of the layer, as in the case of the vitreous selenium layer, or capability they preferably contain a high proportion of photoconductive material in the binder configuration. The requirements of a photoconductive layer containing all or a major proportion of a photoconductive material further restricts the physical characteristics of the final plate, drum or belt in that the physical characteristics such as flexibility and adhesion of the photoconductor to a supporting substrate are primarily dictated by the physical properties -methylphenyl)phenylmethane. the photoconductor, and not by the resin or matrix material which is preferably present in a minor amount.
Another form of a composite photosensitive layer which has also been considered by the prior art includes a layer of photocoductive material which is covered with a relatively thick plastic layer and coated on a supporting substrate.
U.S. Pat. No. 3,041,166 to Bardeen describes such a configuration in which a transparent plastic material overlays a layer of vitreous selenium which is contained on a supporting substrate. In operation, the free surface of the transparent plastic is electrostatically charged to a given polarity. The device is then exposed to activating radiation which generates a hole-electron pair in the photoconductive layer. The electrons move through the plastic layer and neutralize positive charges on the free surface of the plastic layer thereby creating an electrostatic image. Bardeen, however, does not teach any specific plastic materials which will function in this manner, and confines his examples to structures which use a photoconductor material for the top layer.
French Pat. No. 1,577,855 to Herrick et al describes a special purpose photosensitive device adapted for reflex exposure by polarized light. One embodiment which employs a layer of dichroic organic photoconductive particles arrayed in oriented fashion on a supporting substrate and a layer of poly(vinylcarbazole) formed over the oriented layer of dichoric material. When charged and exposed to light polarized perpendicularly to the orientation of the dichroic layer, the oriented dichoric layer and poly(vinylcarbazole) layer are both substantially transparent to the initial exposure light. When the polarized light hits the white background of the document being copied, the light is depolarized, reflected back through the device and absorbed by the dichroic photoconductive material. In another embodiment, the dichroic photoconductor is dispersed in oriented fashion throughout the layer of polyvinyl carbazole.
Shattuck et al, U.S. Pat. No. 3,837,851, discloses a particular electrophotographic member having a charge generation layer and a separate charge transport layer. The charge transport layer comprises at least one tri-aryl pyrazoline compound. These pyrazoline compounds may be dispersed in binder material such as resins known in the art.
Cherry et al, U.S. Pat. No. 3,791,826, discloses an electrophotographic member comprising a conductive substrate, a barrier layer, an inorganic charge generation layer and an organic charge transport layer comprising at least 20 percent by weight trinitrofluorenone.
Belgian Pat. No. 763,540, issued Aug. 26, 1971 (U.S. application Ser. No. 94,139, filed Dec. 1, 1970, now abandoned) discloses an electrophotographic member having at least two electrically operative layers. The first layer comprises a photoconductive layer which is capable of photogenerating charge carriers and injecting the photo-generated holes into a contiguous active layer. The active layer comprises a transparent organic material which is substantially non-absorbing in the spectral region of intended use, but which is "active" in that it allows injection of photo-generated holes from the photoconductive layer, and allows these holes to be transported to the active layer. The active polymers may be mixed with inactive polymers or nonpolymeric material.
Wilson, U.S. Pat. No. 3,542,547, discloses photoconductive elements containing stable organic photoconductors such as triarylmethane leuco bases. More specifically, Wilson discloses a photoconductive element for use in electrophotography comprising a support havng coated thereon a photoconductive insulating layer which comprises an organic photoconductor dispersed in a film-forming insulating resin binder. The photoconductor may be 4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane.
Rule et al, U.S. pat. No. 3,820,989, discloses certain triarylmethane leuco bases which may be used as photoconductive materials dispersed in an insulating resin binder.
Robinson, U.S. Pat. No. 3,533,783, discloses a photoconductive element which comprises a conductive support having coated thereon a layer of a composition comprising a binder, a sensitizer and an organic photoconductor which is overcoated with a layer of a composition comprising a binder and an organic photoconductor. The organic photoconductor may be 4,4'-diethylamino-2,2'-dimethyltriphenylmethane.
Gilman, Defensive Publication of Ser. No. 93,449 filed Nov. 27, 1970, published in 888 O.G. 707 on July 20, 1970, Defensive Publication No. P888,013, U.S. Cl. 96-1.5, discloses that the speed of an inorganic photoconductor such as amorphous selenium, can be improved by including an organic photoconductor in the electrophotographic element. For example, an insulating resin binder may have TiO.sub.2 dispersed therein or it may be a layer of amorphous selenium. This layer is overcoated with a layer of electrically insulating binder resin having an organic photoconductor such as 4,4'-diethylamino-2,2'-dimethyltriphenylmethane dispersed therein.
"Multi-Active Photoconductive Element," Martin A. Berwick, Charles J. Fox and William A. Light, Research Disclosure, Vol. 133; pages 38-43, May 1975, was published by Industrial Opportunities Ltd. Homewell, Havant, Hampshire, England. This disclosure relates to a photoconductive element having at least two layers comprising an organic photoconductor containing a charge-transport layer in electrical contact with an aggregate charge-generation layer. Both the charge-generation layer and the charge-transport layer are essentially organic compositions. The charge-generation layer contains a continuous electrically insulating polymer phase and a discontinuous phase comprising a finely-divided, particulate co-crystalline complex of (1) at least one polymer having an alkylidene diarylene group in a recurring unit and (2) at least one pyrylium-type dye salt. The charge-transport layer is an organic material which is capable of accepting and transporting injected charge carriers from the charge-generation layer. This layer may comprise an insulating resinous material having 4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane dispersed therein.
None of the above mentioned art discloses charge generating material in a separate layer which is overcoated with a charge-transport layer comprising poly(N-vinylcarbazole) which contains an electrically active plasticizer comprising from about 1 to about 25 percent by weight of bis(4-diethylamino-2-methylphenyl)phenylmethane wherein the charge transport material is substantially non-absorbing in the spectral region of intended use, but which is "active" in that it allows injection of photo-generated holes from the charge-generation layer and allows these holes to be transported therethrough. The charge-generating layer is a photoconductive layer which is capable of photo-generating holes and injecting these holes into the contiguous charge-transport layer.
Most importantly, it is well known that when poly(N-vinylcarbazole) is used in a flexible, belt type photoreceptor, it becomes brittle with little or no flexibility after extensive cycling. When normal plasticizers, e.g., dibutylphthalate, are added to plasticize poly(N-vinylcarbazle), the addition of these plasticizers adversely affect the electrical properties of the poly(N-vinylcarbazole). The plasticized poly(N-vinylcarbazole), when used as the transport layer, is not capable of allowing efficient transport of photo-generated holes injected from the photoconductive layer. Therefore, the instant invention overcomes this difficulty by using a plasticizer which is referred to in the instant invention as an electrically active plasticizer. This plasticizer, bis(4-diethylamino-2-methylphenyl)phenylmethane is added in plasticizing amounts, i.e., from abouut 1 to about 25 percent by weight, to the poly(N-vinylcarbazole). The above electrical disadvantages were overcome and the poly(N-vinylcarbazole) was plasticized so that it would remain flexible and non-brittle when used in an application requiring a flexible, photoreceptor which after extended cycling still retains its original electrical properties.